Enhanced catalytic activity in suzuki-miyaura couplings by shell crosslinked pd nanoparticles from alkene-terminated phosphine dendron-stabilized Pd nanoparticles

Article abstract of DOI:10.1246/cl.2012.1700

New alkene-terminated phosphine dendron-protected palladium nanoparticles (NPs) have been synthesized, and the olefin metathesis polymerization of their Pd NPs led to shell crosslinked Pd NPs without aggregation of the Pd NPs. The introduction of crosslinking at the surface of the protective ligand enhanced the catalytic activity of the Pd NPs for SuzukiMiyaura couplings.

Full text of DOI:10.1246/cl.2012.1700

doi:10.1246/cl.2012.1700
1700
Published on the web December 8, 2012
Enhanced Catalytic Activity in Suzuki-Miyaura Couplings by Shell Crosslinked Pd
Nanoparticles from Alkene-terminated Phosphine Dendron-stabilized Pd Nanoparticles
Toshiaki Shiomi, Tsukasa Nakahodo, and Hisashi Fujihara*
Department of Applied Chemistry, Kinki University, Kowakae, Higashi-Osaka, Osaka 577-8502
(Received September 19, 2012; CL-120976; E-mail: h-fuji@apch.kindai.ac.jp)
New alkene-terminated phosphine dendron-protected palla-
dium nanoparticles (NPs) have been synthesized, and the olefin
metathesis polymerization of their Pd NPs led to shell
crosslinked Pd NPs without aggregation of the Pd NPs. The
introduction of crosslinking at the surface of the protective
ligand enhanced the catalytic activity of the Pd NPs for Suzuki-
Miyaura couplings.
O
O
O
O
X
P
O
O
1: X = -CH2O-
Metal nanoparticles (NPs) have been the focus of significant
interest because of their unique electronic, optical, and catalytic
properties.1 Control of the surface properties and reactivity of
the metal NPs is an important aspect of developing nanomaterial
applications. The size, shape, and surface properties of metal
NPs are crucially controlled by the nature of the protective
ligand shells. The preparation of uniform metal NPs has been
intensively pursued because of their applications as nanocata-
lysts in organic transformations such as carbon-carbon bond-
forming reactions.2,3 Specifically, palladium NPs have become
of increasing scientific interest as catalysts for the Suzuki-
Miyaura cross-coupling reaction, which is among the most
powerful methods in organic synthesis.3 As an example, we
reported that optically active 2,2¤-bis(diphenylphosphino)-1,1¤-
binaphthyl-stabilized palladium NPs catalyzed asymmetric
Suzuki-Miyaura cross-coupling reactions at room temperature.4
The development of a generalized method for the fabrication of
multifunctional nanocatalysts remains an important and chal-
lenging issue.
2: X = CH2O(CH2)10O
Figure 1. Structures of 1 and 2. Phosphine dendrons 1 and 2
used to derive the palladium NPs, 1-Pd and 2-Pd NPs.
O
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Cl
Cl
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Ru
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Ph
P(Cy)3
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1-Pd NPs
poly-1-Pd NPs
Scheme 1. Crosslinking of the peripheral alkene groups of
1-Pd NPs by olefin metathesis.
There has been a fundamental interest in the surface
modification of gold NPs by the crosslinking of alkene groups
around the periphery of gold NPs stabilized by thiol ligands with
the alkene groups via alkene metathesis polymerization using
Grubbs’ catalyst.5 Although a number of studies on olefin
metathesis of alkenethiol-stabilized gold NPs and the properties
of shell crosslinked gold NPs have been reported, there is no
report concerning the olefin metathesis of palladium NPs
stabilized by alkene-terminated phosphine dendrons and the
catalytic activity of shell crosslinked palladium NPs.
In our recent research, special emphasis has been placed on
the elucidation of the structural driving forces in order to control
the particle size and shape and the reactivity of the phosphine
dendron-stabilized metal NPs. The key structural elements of the
new phosphine dendrons 1 and 2, having a long alkyl chain at
the focal point in Figure 1, as protective ligands in this study
consist of triphenylphosphine for the coordination site and an
alkene group for the metathesis polymerization site.6 We have
found that the alkene-terminated phosphine dendron-stabilized
palladium NPs, 1-Pd NPs and 2-Pd NPs, have a smaller core
size and narrower size distribution (2.2 « 0.3 nm for 1-Pd NPs
and 1.9 « 0.3 nm for 2-Pd NPs) and that the olefin metathesis
polymerization of the 1-Pd NPs or 2-Pd NPs led to the shell
crosslinked palladium NPs, poly-1-Pd NPs and poly-2-Pd NPs,
without aggregation of the palladium NPs (Scheme 1). Ideally,
the shell crosslinked Pd NPs should be surrounded by a
nonadherent coating, or nanocapsule, that prevents aggregation
of the Pd NPs, permits passage of the catalyst substrate and
product, but does not adversely affect the catalytic properties.
These properties could promote the activity of the nanoparticle
catalysts. Interestingly, we have found that the noncrosslinked
Pd NPs, 1-Pd NPs and 2-Pd NPs, and the shell crosslinked Pd
NPs, poly-1-Pd NPs and poly-2-Pd NPs, showed very different
catalytic activities for the Suzuki-Miyaura cross-coupling
reactions, although they have similar diameters of 1.9-2.2 nm.
Generally, the catalytic activities of metal NPs depend on the
particle size. This communication presents the fabrication of
shell crosslinked Pd NPs, poly-1-Pd NPs and poly-2-Pd NPs, as
multifunctional metal-incorporated nanocapsules with enhanced
catalytic activities for Suzuki-Miyaura couplings by shell
crosslinked Pd NPs.
A typical procedure for the preparation of the Pd NPs, 1-Pd
NPs, is as follows: To a vigorously stirred solution of K2[PdCl4]
(65 mg, 0.2 mmol) in 7 mL of deionized water was added
phosphine 1 (330 mg, 0.4 mmol) in 30 mL of THF. NaBH4
Chem. Lett. 2012, 41, 1700-1702
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a
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20 nm
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Particle Diameter/nm
Particle Diameter/nm
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b
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20 nm
10 nm
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Particle Diameter/nm
Particle Diameter/nm
Figure 2. (a) TEM image and distribution of 1-Pd NPs
(2.2 « 0.3 nm). (b) TEM image and distribution of poly-1-Pd
NPs (2.1 « 0.3 nm).
Figure 3. (a) TEM image and distribution of 2-Pd NPs
(1.9 « 0.3 nm). (b) TEM image and distribution of poly-2-Pd
NPs (1.9 « 0.3 nm).
(80 mg, 2 mmol) in 5 mL of deionized water was then added.
The mixture was stirred for 1 h at room temperature. After the
reaction, the filtrate was evaporated in vacuo to yield the 1-Pd
NPs. Purification (dichloromethane-diethyl ether) of the 1-Pd
NPs was repeated until no free phosphine remained, as detected
Me
Br + (HO)2B
(1)
Me
3
4
5
H3C(H2C)11
O
Br + (HO)2B
H3C(H2C)11
O
(2)
7
6
1
by TLC and H and 13C NMR spectroscopy. The 1-Pd NPs are
remarkably stable both in solution and the solid state. The 2-Pd
NPs were prepared by the same procedure.
Scheme 2. Suzuki-Miyaura coupling reactions.
The particle size and size distribution of the 1-Pd NPs and
2-Pd NPs were analyzed by transmission electron microscopy
(TEM). The core sizes of the NPs are small i.e., 2.2 « 0.3 nm for
the 1-Pd NPs (Figure 2a) and 1.9 « 0.3 nm for the 2-Pd NPs
(Figure 3a). The X-ray photoelectron spectroscopy (XPS)
spectra of the 1-Pd NPs and 2-Pd NPs show that the binding
energies for the Pd 3d doublet are 336.0 and 341.3 eV. There is a
feature at 285 eV from C 1s and one at 132 eV from P 2p. There
are no obvious surface plasmon bands in the UV-vis spectra of
the 1-Pd NPs or 2-Pd NPs in CH2Cl2. The absence of a plasmon
band for the 1-Pd NPs or 2-Pd NPs agrees with theoretical
predictions7 and experimental observations for 2.2 nm alkane-
thiolate-stabilized Pd NPs8 but not with the report of a 302 nm
surface plasmon band for 2.2 nm octadecanethiolate-protected
Pd NPs.9
The crosslinking of the alkene groups around the periphery
of the 1-Pd NPs or 2-Pd NPs was achieved by Grubbs’
metathesis as follows (Scheme 1):5,10 To a mixture of 1-Pd NPs
(40 mg) in dry benzene (5 mL) was added a solution of the first
generation Grubbs’ catalyst11 (20 mg) in dry benzene (95 mL)
under an Ar atmosphere. The resulting mixture was stirred for
48 h at room temperature. After the reaction, the shell cross-
linked palladium NPs, poly-1-Pd NPs, were purified using
dichloromethane-hexane. Following the metathesis, the NPs
remained soluble in benzene, dichloromethane, chloroform,
tetrahydrofuran, acetone, ethyl acetate, dimethylformamide,
dimethyl sulfoxide, and methanol. The poly-1-Pd NPs were
1
analyzed by TEM and H NMR spectroscopy. The TEM image
of the poly-1-Pd NPs shows a core size of 2.1 nm, which
indicates no aggregation of the palladium cores during the
metathesis polymerization (Figure 2b). Analogously, no aggre-
gation for the metathesis of the 2-Pd NPs was observed, as
evidenced by the TEM image (Figure 3b). After the metathesis
1
of the 1-Pd NPs, the H NMR spectrum in Figure S1a6 shows
that the terminal alkene proton resonances at ¤ ca. 5.1 and 5.8 as
broad signals of 1-Pd NPs disappear and/or diminish and that
a new broad peak appears at ¤ ca. 5.5 (Figure S1b),6 which
corresponds to the internal alkene formed by the metathesis
reaction.5 These findings indicate that crosslinking of the double
bonds on the nanoparticle surface occurred. Similar 1H NMR
data were obtained for the poly-2-Pd NPs.
The catalytic activities of the palladium NPs (Ph3P-
stabilized Pd NPs, 1-Pd NPs, poly-1-Pd NPs, 2-Pd NPs, and
poly-2-Pd NPs) dispersed in an organic solvent were tested in
a very important carbon-carbon bond-forming reaction, the
Chem. Lett. 2012, 41, 1700-1702
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

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Table 1. Suzuki-Miyaura couplings catalyzed by Pd NPs:
Ph3P-Pd, 1-Pd, poly-1-Pd, 2-Pd, and poly-2-Pd NPsa
difference in the catalytic activities between the noncrosslinked
and crosslinked Pd NPs. We are currently investigating the
fundamental reasons for the observed differences in the catalytic
activities.
Catalyst,
Boronic
acid
Yield
/%
Entry
Halide
Product
NPs
In summary, shell crosslinked Pd NPs, poly-1-Pd NPs, were
found to be versatile catalysts for the Suzuki-Miyaura coupling
reactions under mild conditions. Remarkable differences in the
catalytic activities of the crosslinked and noncrosslinked Pd NPs
were found in the Suzuki-Miyaura couplings, although the Pd
NPs have similar diameters. This aspect may have significant
implications for the use of shell crosslinked Pd NPs as catalysts
in organic reactions.
1
2
3
4
5
6
7
8
9
Ph3P-Pd
1-Pd
poly-1-Pd
2-Pd
poly-2-Pd
1-Pd
poly-1-Pd
2-Pd
3
3
3
3
3
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7
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19b
33
86
38
62
37
93
34
71
poly-2-Pd
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas (Nos. 21108522 and 23108723,
“pi-Space”) and a Scientific Research, No. 21350030, from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan.
aReaction conditions: 1.0 mmol halide, 1.5 mmol boronic acid
(4), 3 mmol KF, 0.01 mol % of Pd NPs, THF, 50 °C, and 20 h.
bPh3P-stabilized Pd NPs.
References and Notes
Suzuki-Miyaura coupling reaction. This choice was based on
the fact that such a reaction provides a powerful tool for the
synthesis of biaryls, which are found in many natural and
synthetic products. Although it is known that Suzuki-Miyaura
coupling reactions are carried out at reflux, we have carried out
the Suzuki-Miyaura coupling reactions at 50 °C (Scheme 2).12
Typically, a mixture of 4-bromotoluene (3: 1.0 mmol) and
phenylboronic acid (4: 1.5 mmol) in the presence of the
triphenylphosphine (Ph3P)-stabilized Pd NPs (diameter: 2 nm,
0.01 mol %) and KF (3 mmol) in anhydrous THF (2 mL) was
stirred at 50 °C under argon for 20 h (Scheme 2-1). After work-
up, the crude products were purified by silica gel column
chromatography (n-hexane:benzene = 1:1) to give the coupling
product 5 in 19% yield (Table 1). The yield of the coupling
product 5 increased to 33% when the 1-Pd NPs (diameter:
2.2 nm) were used as a catalyst (Table 1).13 In contrast to the 1-
Pd NPs, a more significant improvement in the catalytic activity
can be achieved by the shell crosslinked Pd NPs, poly-1-Pd
NPs, i.e., the coupling product 5 was obtained in 86% yield
using the poly-1-Pd NPs (diameter: 2.1 nm) (Table 1), although
they have similar core sizes. Most notably, the yields in the
reaction of 3 with 4 were remarkably influenced by the structure
of the protective ligand on the noncrosslinked (1-Pd NPs) and
crosslinked (poly-1-Pd NPs) NPs, even though the Pd NPs have
very similar diameters (2.2 nm for 1-Pd NPs and 2.1 nm for
poly-1-Pd NPs). Moreover, we found a similar catalytic activity
of the 1-Pd NPs and 2-Pd NPs for the Suzuki-Miyaura
couplings of 3 with 4 (Table 1); however, the poly-2-Pd NPs
showed less catalytic activity than that of the poly-1-Pd NPs
(Table 1). The above catalytic activities for the reaction of 3
with 4 were similar to the results from the reaction of 6 with 4
using those Pd nanoparticle catalysts (Table 1). The shell
crosslinked Pd NPs, poly-1-Pd NPs, acted as good catalysts
for the bromide 6, having a longer alkyl-chain than the bromide
3, i.e., the reaction of 6 with 4 in the presence of poly-1-Pd NPs
afforded the coupling product 7 in 93% yield under relatively
mild conditions such as at 50 °C without reflux (Scheme 2-2)
(Table 1). Notably, these findings indicate that the catalytic
activities of the crosslinked Pd NPs, poly-1-Pd NPs and poly-2-
Pd NPs, are higher than those of the noncrosslinked Pd NPs, 1-
Pd NPs and 2-Pd NPs. Thus, we have found a considerable
1
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Chem. Lett. 2012, 41, 1700-1702
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/